1. Field of the Invention
The present invention relates to a magnetic carrier for an electrophotographic developing agent. More specifically, the invention relates to a magnetic carrier for an electrophotographic developing agent which is free from carrier trail, exhibits stable charging property of the toner, and makes it possible to form image of high density and high quality for extended periods of time.
2. Description of the Prior Art
In forming image by the electrophotographic method, two-component magnetic developing agents consisting of a toner and a magnetic carrier have been extensively used. The magnetic carriers can be roughly divided into two kinds, i.e., magnetic particle carriers such as iron powder and sintered ferrite particles, and so-called binder carriers obtained by granulating the magnetic powder together with a binder resin.
Japanese Laid-Open Patent Publication No. 324456/1992 discloses a carrier for a two-component developing agent consisting of at least a binder resin and a magnetic powder, the content of the magnetic powder being from 85 to 99% by weight of the weight of the binder resin, and the carrier gradually collapsing at the time of developing under the load of not larger than 100 g/cm.sup.2 This carrier gradually collapses in the developing device and is consumed together with the toner. Therefore, the toner concentration in the developing agent is maintained constant at all times, and the device can be realized in a small size since there is no need of controlling the toner concentration in the developing agent.
In the above-mentioned prior art, the carrier-collapsing property is imparted by adjusting the content of magnetic powder in the carrier to lower the collapsing load. However, the carrier does not necessarily collapse uniformly, and sizes of the particles formed by the collapse undergo variation to a considerable degree, and the carrier adheres onto the non-image portions and fogging develops on the background of the image to deteriorate picture quality.
On the other hand, the two-component developing agent has a problem in regard to the occurrence of a so-called spent toner by which toner particles which are a resin composition containing a charge control agent gradually migrate like a film onto the surfaces of the carrier particles. When such a filming develops, the electric charge of the toner loses stability resulting in the occurrence of such problems as a decrease in the image density and fogging.
In an amorphous silicon (a-Si) photosensitive material, furthermore, the surfaces are attacked by discharge products such as ozone and the like produced by the corona charging, whereby oxides are formed rendering the surfaces to become hydrophilic and developing such defects as image flow and the like. To prevent such defects, a polishing agent has heretofore been contained in the developing agent.
In the binder-type magnetic carrier used for the two-component developing agent, if the magnetic powder can be peeled off the surfaces of the carrier maintaining controlled diameter and amount, fresh surfaces can be exposed on the surfaces of the carrier particles, and the charging property does not become unstable due to toner filming. In the case of the amorphous silicon photosensitive material, furthermore, it can be expected that the particles peeled off the surfaces of the carrier polish the oxide layer on the surface of the photosensitive material.